Fig. 9. Working model for retrograde sorting on the early endosome. (A) In control cells, clathrin organizes microdomains with which Shiga-toxin associates (step 1). We hypothesize that microdomain construction induces initial membrane-curvature changes leading to tube formation. A retromer-dependent process then allows stabilizing and processing of membrane curvatures, leading to the formation of retrograde transport intermediates (step 2). (B) In clathrin-depleted cells, the early endosomal retrograde sorting domain cannot form any more. Under these conditions, retrograde transport to TGN/Golgi membranes is inhibited and Shiga toxin distributes throughout the early endosomal pathway, perfectly overlapping with TfR. (C) In Vps26-depleted cells, retrograde transport intermediates cannot form and STxB remains blocked in retrograde tubules that are devoid of TfR. For further details see text.